The present invention relates to a backrest device in a chair and particularly relates to a backrest device in a chair comprising a flexible back plate for supporting the back of an occupant in a back frame supported by a leg.
Such a backrest device in a chair is known from JP2002-125797A and JP2005-160558A.
The backrest device in the publications disclose that a back frame and a back plate have nearly uniform thickness to make it impossible for an upper part of the back plate onto which the back of an occupant strongly presses to be bent rearward effectively. So the chair would not be comfortable to sit on.
In order to allow the back of the occupant to fit over the backrest, the whole back plate may preferably be bent to be convex. But it would be very hard to bend a uniform-thickness back plate only by forming a number of opening in the back plate.
A number of openings close to each other in the back plate would decrease bending rigidity or strength of the back plate to reduce durability.
In order that side back frames may firmly be mounted to back rods fixed to legs and that the back plate pressed by the back of the occupant in the back frame may effectively be bent rearward to get more comfortable to sit in, JP2002-125797A discloses a backrest device with side back frames having a relatively large circular cross-section to provide higher bending rigidity.
In contrast to the thinner back plate, the thicker back frame provides poor appearance.
JP2005-160558A discloses that the side back frames in FIG. 10 have a circular cross-section for improving bending rigidity, but causes poor appearance. The back frame in FIG. 12 has a flat cross section and provides lower bending rigidity. When the inner back plate is pressed rearward, the back frame is bent rearward together. So the back plate would be unlikely to bend rearward.